Method of heat-treating aluminum and articles thereby



Patented 12, 1932 UNITED STATES PATENT: OFFICE i.

mun w. names, or rnnnroar, rmmvuu, Assmuon r summon coll- PAIN'Y'. OF AMERICA, OF PITTSBURGH, SYLV'ANLL PENNSYLVANIA, A CORPORATION OF ramp 1 METHOD or near-Tammie nm'nmimr m Annex-rs N 0 Drawing.

The invention relates to the heat treatment of aluminum and aluminum base alloys in fused salt baths, and to aluminum and aluminum base articles resulting therefrom, both as aluminum and-such alloys being hereinafter designated as aluminum.

The solution or other thermal treatment of aluminum alloys often requires, for satisfactory results, heated to the heat treatment temperature, at which it is held for a period of time and then rapidly cooled as by quenching in water. It 1s common practice to use as a heating me dium a fused salt bath which mayreadily be maintained at the temperature-desired.

A serious disadvantage of heat treating aluminum-in fused salt baths is the corrosion which may take place on the surface of the metal during or after quenching in water.

This is particularly marked where the .fused salt used is an alkali metal nitrate, such as SOdlllIIl nitrate, which is commonly used for such purposes. The exact nature of the corrosive action a matter about which there may besome question, but the known result is the formation of a white powder (probably composed of oxides) on the surface of the metal. This powder is produced in a loosely adherent form on the surface of the article thus treated, constituting a destructible coating capable of being rubbed off.

The corrosive action just described, commonly referred to in the art as staining, is objectionable, particularly in lthe' case of ly worked, as by rolling, in which case the foreign matter formed on the surface of the metal'injures its surface Numerous attempts have been made to overcome difiiculty, but none have-been attendediwith any appreciable degree of success. Forexample, recognizing that the alkalinity of a' that the article be quickly aluminum articles which are to be subsequentsuch purity are expensive and difiicult to ob- Application filed February 80, 1881. Serial No. 617,817.

twin, and alkali metal nitrates, regardless of initial alkalinity, become more alkaline when held ina state of fusion.

An object of this invention is to provide a method of heat treating aluminum in a fused salt bath, which method obviates the foregoing dilficulties, and which not only substantially eliminates corrosive staining of the sheet or article being treated, but also forms a surface on it which is resistant to corrosion in the use of the article,

Another object is to provide aluminum articles free, or substantially so, from heattreating corrosion stains, and having surfaces which are resistant to corrosion.

The invention ispredicated upon my discovery that by heat treating aluminum in a fused salt bath containing an oxygen compound of chromium the surface of the alumi num is not merely free, or substantially free, from corrosive staining, but is rendered resistant to subsequent corrosion. The oxygen compound of chromium may be added'as a chromic or dichromic acid, or as a salt or salts thereof, either singly or in combination, and

the thus modified used bath used for the heat treatment of aluminum in the same manner as other fused baths have been used. Thefused salt bath now in general use for heat treating aluminum consists of alkah metal nitrates, such as sodium nitrate, potassium nitrate, or a mixture of the two, but I prefer to use sodium nitrate chiefly because. of the commercial consideration of cost. The effects of the addition of an oxygen compound of chromium are to inhibit corrosion throughout the heat treating cycle, including the quenching step, and to render the surfaces of the heat treated articles resistant to corrosion.

ill have found that when a chromic acid salt is added to a fusedznitrate bath it serves the dual function of neutralizing the bath'and inhibiting the tendency of the nitrate to become alkaline when held in a state of fusion. It is not necessary that, prior to making the addi tion, the bath be completely neutralized, because the addition of small amounts has been found to exert some inhibitive action upon corrosion even before the bath has been completely neutralized, but I prefer to use an able to add a mixture of chromates and dichromates, with or ithout the addition of chromic or dichromic acid, in all cases corrosion of the surface of the metal is efi'ectively prevented. If a chromate be added, it is usually necessary to use a much greater quantity in order to secure the desired inhibiting effect than where a dichromate is used, although this depends on the grade of nitrate which is used for the bath.

When, in the practice of this invention, it is desired to utilize a fused heat treating bath comprising a nitrate made from Chilean saltpeter containing small amounts of impurities such as chlorides, sulfates, iodates, etc.,' vand being slightly basic in character, the

amount of, chromic acid salt required to neutralize the alkalinity of the nitrate and inhibit subsequent corrosion of the metal treated therein may be determined by titrating the fused salt according to well known analytical procedure. As an example of the use of. this procedure to determine the proper quantity of npotassium dichromate to be added to a ta containing 30,000 pounds of nitrate to be v used for the heat treatment of strong aluminum base alloy sheet, samples of nitrate of a given weight were fused with varying quantities of potassium dichromate, the alkalinity of water solutions of these samples being determined. It was noted that with. the addition of one-half of one per cent of potassium dichromate the aqueous solution of the fused salt mixture was acid in reaction, and it was further determined that with the addition of one-half of one per cent of potassium dichromate there was no visible corrosion. On the basis of this test it was determined that 150 pounds of potassium dichromate should be added to the 30,000-pound tank of molten nitrate. The addition may be, and from the standpoint of safety preferably is, made to the nitrate before it is fused. V

The amount of an oxygen compound of chromium required depends on various factors, such as the particular com ound or compounds used, the alkalinity an grade of the nitrate, extent of the heat treating'cycle, temperature of the bath, and the character and composition of the metal being treated, but in every case a simple experiment, such as that just explained, maybe used to determine theamount required. Thereafter, the heat treating is conducted in the usual way by immersing the aluminum in the fused bath heated to the desired temperature.

The practice of my improved method of heat treating aluminum in a modified fused salt eliminates, or reduces within small and practical limits, the corrosion and pitting of the surface of the metal and the formation of loosely adherent oxides thereon, and, in addition to this, renders the surface of the metal resistant to corrosion. There is also an important attending advantage in the improvement in the appearance of the metal treated, and, in the case of sheet which is to be subjected to a subsequent rolling operation, a substantial advantage incident to the elimination of foreign matter which becomes trapped between the rolls and the metal and mars the surface of the sheet. Still further advantages of the invention are that it makes possible the use of inferior grades of nitrate otherwise impracticable for use as fused baths for the heat treatment of aluminum, and that it reduces the usual care required in washing sheet aluminum after its heat treatment. While the practice of my invention is particularly advantageous where inferior grades of alkali metal nitrates are used as fused heat treating baths, it is also advantageous where the grades of nitrates used are approximately neutral, it having been found that the use of such grades frequently causes severe corrosion.

According to theprovisions of the patentstatutes, I have explained the principle and mode of operation of my invention, and have specifically described how it may be practiced. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

' I claim as my invention:

1. The method of heat-treating aluminum,

comprising heating it in a fused salt bath containing an oxygen compound of chromium in an amount suflicient to substantially inhibit heat-treating corrosion.

2. The method of heat-treating aluminum, comprising heating it in a fused salt bath containing a chromium acid salt in an amount s'ufiicient to substantially inhibit heat-treating. corrosion.

3. The method of heat-treating aluminum, comprising heating it in a fused salt bath containing a dichromate in an amount sufficient to substantially inhibit heat-treating corrosion.

4. The method of heat-treating aluminum,

comprising heating it in a fusedbath of sodium nitrate containing a chromic acid saltin an amountsuflicient to substantially inhibit heat-treating corrosion.

5. The method of heat-treating aluminum, comprising heating it in a fused bath of sodium nitrate containing potassium dichromate in an amount suflicient to substantially inhibit heat-treating corrosion.

' 6. The method of heat-treating aluminum,

comprising heating it in a fused bath of sodium nitrate containing about one-half of one per cent of potassium dichromate in an amount suflicient to substantially inhibit heat-treating corrosion.

In testimony whereof, I si my name.

- RALPH V BRlIDGEs. 

